1. Field of the Invention
This invention relates to aluminum alloys, and specifically to high tensile strength aluminum-silicon (Alxe2x80x94Si) hypereutectic alloy suitable for high temperature applications such as heavy-duty pistons and other internal combustion applications. It relates particularly to a process for producing cast articles from this high tensile strength and high wear resistance Alxe2x80x94Si hypereutectic alloy.
2. Discussion of the Related Art
Alxe2x80x94Si casting alloys are the most versatile of all common foundry cast alloys in the production of pistons for automotive engines. Depending on the Si concentration in weight percent, the Alxe2x80x94Si alloy systems fall into three major categories: hypoeutectic ( less than 12 wt. % Si), eutectic (12-13 wt. % Si) and hypereutectic (14-25 wt. % Si). In hypereutectic alloys, Si plays an important role by enhancing the cast article""s surface hardness and wear resistance properties more than hypoeutectic and eutectic alloys. High silicon content in hypereutectic alloys also results in higher elastic modulus and lower thermal expansion. Currently, hypereutectic Alxe2x80x94Si alloys are crucial for high wear resistance applications such as pistons and reciprocate connecting rods. However, conventional hypereutectic alloys, such as 390, are not suitable for high temperature applications, such as in the automotive field, because their mechanical properties, such as tensile strength, are not as high as desired in the temperature range of 500xc2x0 F.-700xc2x0 F. Above an elevated service temperature of about 450xc2x0 F., the major alloy strengthening phases such as the xcex8xe2x80x2 (Al2Cu) and Sxe2x80x2 (Al2CuMg) will precipitate rapidly, coarsen, or dissolve, and transform themselves into the more stable xcex8 (Al2Cu) and S (Al2CuMg) phases. The undesirable microstructure and phase transformation results in drastically reduced mechanical properties, more particularly the ultimate tensile strength and high cycle fatigue strengths, for hypereutectic Alxe2x80x94Si alloys.
One approach taken by the art is to use ceramic fibers or particulates to increase the strength and improve wear resistance of Alxe2x80x94Si alloys as a substitute for conventional hypereutectic alloys.
This approach is known as the aluminum Metal Matrix Composites (MMC) technology. For example, R. Bowles has used ceramic fibers to improve tensile strength of 332.0 alloy, in a paper entitled, xe2x80x9cMetal Matrix Composites Aid Piston Manufacture,xe2x80x9d Manufacturing Engineering, May 1987. Moreover, A. Shakesheff has used ceramic particulates for reinforcing another type of A359 alloy, as described in xe2x80x9cElevated Temperature Performance of Particulate Reinforced Aluminum Alloys,xe2x80x9d Materials Science Forum, Vol. 217-222, pp. 1133-1138 (1996). In a similar approach, cast aluminum MMC for pistons using a eutectic alloy such as the 413.0 type, has been described by P. Rohatgi in a paper entitled, xe2x80x9cCast Aluminum Matrix Composites for Automotive Applications,xe2x80x9d Journal of Metals, April 1991.
Another approach taken by the art is the use of the Ceramic Matrix Composites (CMC) technology in the place of Alxe2x80x94Si alloys. For example, W. Kowbel has described the use of non-metallic carbonxe2x80x94carbon composites for making pistons to operate at high temperatures in a paper entitled, xe2x80x9cApplication of Net-Shape Molded Carbonxe2x80x94Carbon Composites in IC Engines,xe2x80x9d Journal of Advanced Materials, July 1996. Unfortunately, the material and processing costs of these MMC and CMC technologies are substantially higher than those for conventional casting, and they therefore cannot be considered for large usage in mass production, such as engine pistons.
A primary object of the present invention is to provide a process for making a cast article from an aluminum alloy, which cast article has improved mechanical properties at elevated temperatures.
According to the present invention, an aluminum alloy having the following composition, by weight percent, is first provided:
In this aluminum alloy the ratio of Si:Mg is 15-35, preferably 18-28, and the ratio of Cu:Mg is 4-15.
An article is then cast from this composition, and the cast article is aged at a temperature within the range of 400xc2x0 F. to 500xc2x0 F. for a time period within the range of four to 16 hours.
In a particularly preferred embodiment, after the article is cast from the alloy, the cast article is first heat treated in a specifically-defined solutionizing step which dissolves unwanted precipitates and reduces any segregation present in the alloy. After this solutionizing step, the cast article is quenched, and is subsequently aged at an elevated temperature for maximum strength.